4 S (s) + 8 NaOH (aq) → 3 Na2S (aq) + Na2SO4 (aq) + 4 H2O (l)
This is an oxidation-reduction (redox) reaction:
3 S0 + 6 e- → 3 S-II (reduction)
S0 - 6 e- → SVI (oxidation)
S is both an oxidizing and a reducing agent (disproportionation (dismutation)).